Inorganic substrates have been coated with compositions that contain pigments to impart color properties to the substrate for aesthetic purposes. The coated substrates are generally applied or affixed to specific carriers to provide a desired color to the object. For example, coated inorganic granules are often utilized on granule-surfaced bituminous roll roofing and asphalt shingles. The granules, which are partially embedded in one surface of asphalt-impregnated shingles or asphalt-coated fiber sheet material, form a coating which provides an inherently weather-resistant, fire resistant, and decorative exterior surface. The layer of roofing granules functions as a protective layer to shield the bituminous material and the base material from both solar (e.g. ultraviolet radiation) and environmental degradation.
Inorganic substrates are generally coated by applying a slurry containing an inorganic binder and pigment particles onto the substrate. In granular form, the inorganic material is heated in a rotary kiln and mixed with the slurry of inorganic binder and pigment particles. The coated inorganic granules are first dried and then fired at temperatures in excess of 170° C. to insolubilize the binder. The resulting coated granule has a hardened coating that exhibits a selected coloring due to the inclusion of the pigments.
Coated granules are often produced and selected to provide a desirable color to a finished structure or building. It is desirable that the color be consistent over time in order to maintain the appearance of the building. Discoloration of roofing shingles and other building materials due to algae infestation is has become especially problematic in recent years. Algae tend to grow on building materials in areas where moisture is retained. Discoloration has been attributed to blue-green algae, Gloeocapsa magma, transported as air-borne particles. The infestation may be particularly acute on asphalt shingles.
It would be an advantage to provide a coating composition that is capable of preventing algal growth on building materials which incorporate coated inorganic substrates.